Based on the routine and non-conventional data, mesoscale convective systems and backgrounds causing heavy rain in Sichuan and Chongqing during 3—6 September 2004 are analyzed. It is found that (1) the reason for heavy rainfall is meso-β scale convective systems, occurred and developing frequently on the low vortex and shear line, with local and explosive features occurred and developed in the Sichuan Province and Chongqing City; (2) the lower-level convergence and upper-level divergence and vertical ascent motion probably provided the suitable dynamic environment for the generation and development of the mesoscale convective system; and (3) strong convergence of water vapor flux and the conditional convective instability in the lower troposphere probably provided the suitable thermodynamic environment for the generation and development of the mesoscale convective system.